Busses for bifacial photovoltaic cells

ABSTRACT

PV module composed of individual PV cells oriented and electrically connected according to a methodology that is viable for at least i) bifacial cells with substantially equal solar-energy conversion efficiency achievable on each side of each cell, and ii) PV modules with low operating current. Embodiments of the invention facilitate the use of different busing technologies to reduce cost and complexity of the resulting PV module while increasing the electrical energy harvested by the PV module.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims benefit of and priority from U.S.Provisional Patent Application Nos. 61/559,425 filed on Nov. 14, 2011and titled “Advanced Bussing Options for Equal Efficiency BifacialCells”; 61/559,980 filed on Nov. 15, 2011 and titled “FlexibleCrystalline PV Module Configurations; 61/560,381 filed on Nov. 16, 2011and titled “Volume Hologram Replicator for Transmission Type Gratings”;and 61/562,654 filed on Nov. 22, 2011 and titled “Linear ScanModification to Step and Repeat Holographic Replicator”. The disclosureof each of the abovementioned Provisional Patent Applications isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates generally to conversion of solar energy toelectrical energy and, more particularly, to ways of orienting andelectrically coupling of bifacial photovoltaic (PV) cells.

BACKGROUND OF THE INVENTION

Solar energy will satisfy an important part of future energy needs.While the need in solar energy output has grown dramatically in recentyears, the total output from all solar installations worldwide stillremains around 7 gigawatts, which is only a tiny fraction of the world'senergy requirement. High material and manufacturing costs, low solarmodule efficiency, and shortage of refined silicon limit the scale ofsolar power development required to effectively compete with the use ofcoal and liquid fossil fuels.

The key issue currently faced by the solar industry is how to reducesystem cost. The main-stream technologies that are being explored toimprove the cost-per-kilowatt of solar power are directed to (i)improving the efficiency of a solar cells that comprise solar modules,and (ii) delivering greater amounts of solar radiation onto the solarcell. In particular, these technologies include developing thin-film,polymer, and dye-sensitized photovoltaic (PV) cells to replace expensivesemiconductor material based solar cells, the use high-efficiencysmaller-area photovoltaic devices, and implementation of low-costcollectors and concentrators of solar energy.

While the reduction of use of semiconductor-based solar cells is showinggreat promise, for example, in central power station applications,challenges for the use of conventional solar cells remain forresidential applications due to the form factor and significantly higherinitial costs. Indeed, today's residential solar arrays are typicallyfabricated with silicon photovoltaic cells, and the silicon materialconstitutes the major cost of the module. Therefore techniques that canreduce the amount of silicon used in the module without reducing outputpower will lower the cost of the modules.

The use of devices adapted to concentrate solar radiation on a solarcell is one of such techniques. Various light concentrators have beendisclosed in related art, for example a compound parabolic concentrator(CPC); a planar concentrator such as, for example, a holographic planarconcentrator (HPC) including a planar highly transparent plate and aholographically-recorded optical element mounted on its surface; and aspectrum-splitting concentrator (SSC) that includes multiple, singlejunction PV cells that are separately optimized for high efficiencyoperation in respectively-corresponding distinct spectral bands. Aconventionally-used HPC is deficient in that the collection angle,within which the incident solar light is diffracted to illuminate thesolar cell, is limited to about 45 degrees. Production of a typical SSC,on the other hand, requires the use of complex fabrication techniques.

Historically, PV cells have been monofacial, meaning that they have asingle active surface capable of converting incident solar radiation toelectric potential. Historically, monofacial solar cells are fabricatedwith a film stack including an anti-reflective/hard coating optimizedfor transmittance at wavelengths for which silicon has the highestquantum efficiency, passivation, n doped and p doped silicon forming asingle p-n junction, and a back electrode. Conventionally, the backelectrode is a layer of metal such as aluminum. The front electrode isconventionally provided by a layer of transparent conductive materialsuch as Indium Tin Oxide (ITO) in contact with a higher conductivitysmall area electrode made of Al, Ag, or some other metal or alloy. Sinceeach conventional monofacial solar cell generates about 0.5V underillumination, conventional monofacial solar cells are typically arrangedin electrical series, with the back electrode of a first cellelectrically coupled to the front electrode of a second (i.e., adjacent)cell (or vice-versa). This series connection is repeated until thedesired voltage is obtained.

Relatively recently, bifacial solar cells have been fabricated, whichhave photovoltaically active regions on both the front and back sides.Certain conventional bifacial solar cells are fabricated as an n+−p−p+stack between front and back electrodes. Other configurations arepossible, so long as there are two junction regions proximate to a frontactive surface and back active surface, where each junction region formsan electron-hole pair. The front and back electrodes are conventionallyfabricated from a transparent conductor like ITO in electrical contactwith small area metal electrode (i.e., a bus bar or finger).Historically bifacial solar cells have had unequal efficiency betweenthe front and back sides of the cells. Accordingly, conventionalindividual bifacial cells, when assembled into panels or series, are alloriented such that the “front” or high efficiency side is oriented tointercept direct sunlight, while the lower efficiency or “back” side isoriented to receive indirect sunlight from scatter, reflection off theground or mounting surface, for example. Such orientation and associatedelectrical connection between and among the cells does not allow tomaximize the electrical energy output from the resulting panels. PVmodules or panels that take advantage of different orientation of andelectrical connections among the individual bifacial PV cells is,therefore, required.

SUMMARY OF THE INVENTION

Embodiments of the invention are directed toward systems and methods forbussing bifacial solar (or photovoltaic, PV) cells, and/or solar cellshaving relatively low operating current. In one embodiment, bifacialsolar cells having substantially equal conversion efficiency are laidout in an alternating fashion to form an array, with sides of thesecells corresponding to alternating electrical polarity facing the samedirection. Moreover, the fronts and the backs of so laid out alternatingcells are bussed together to form an array of the PV cells. In relatedembodiments of such PV-cell arrays, the component bifacial cells are cutor diced into smaller areas, such that each reduced area cell has anassociated current level reduced in comparison with a bigger sizebifacial cell.

Embodiments of the invention provide a solar module comprising (i) afirst bifacial solar cell having a first front active side and a firstback active side, wherein the first front and back active sides havesubstantially equal solar conversion efficiencies, the first frontactive side having a first electrical polarity, the first back activeside having a second electrical polarity that is opposite to the firstelectrical polarity; (ii) a second bifacial solar cell having a secondfront active side and a second back active side, wherein the secondfront and back active sides have substantially equal solar conversionefficiencies, the second front active side having the second electricalpolarity and the second back active side having the first electricalpolarity. The orientation of the first and second bifacial cells in themodule is such that the first and second bifacial solar cells orientedin series with the first front active side facing in substantially thesame direction as the second front active side. The module furtherincludes a bus bar electrically coupling the first front active side tosecond front active side. In one embodiment, the first and secondbifacial solar cells are arranged adjacent to one other, and the firstfront active side is substantially coplanar with the second front activeside. In a related embodiment, the module may additionally include athird bifacial solar cell having a third front active side and a thirdback active side, wherein the second front and back active sides havesubstantially equal solar conversion efficiencies, the third frontactive side having the first electrical polarity, the third back activeside having the second electrical polarity. The third bifacial solarcell is oriented, in the module, such that the third front active sidefaces substantially the same direction as the first front active side. Abus bar is added to electrically couple the second back active side tothe third back active side. Furthermore, an encapsulant layer may bedisposed on the first front active side and the second front active sidesuch as to cover the bus bar.

Embodiments of the invention also provide a method for fabrication of aphotovoltaic (PV) module. The method includes at least (i) separating aPV cell having an original size into a plurality of PV sub-cells, eachsub-cell having a size smaller than the original size; and (ii)electrically coupling the sub-cells in series such that a first side ofthe first sub-cell having a first electrical polarity is electricallyconnected to a first side of the second sub-cell having a secondelectrical polarity, the first side of the first sub-cell and the firstside of the second sub-cell oriented to face substantially the samedirection. The method may additionally include positioning the firstside of the firs sub-cell to be substantially co-planar with the firstside of the second sub-cell. In one embodiment, the step of electricallycoupling includes a) depositing a conformable electrically conductivematerial on a first surface of an optically-transparent encapsulationlayer; and b) covering the first and second sub-cells with theoptically-transparent encapsulation layer such that the first surface ofthe optically transparent encapsulation layer carrying the conformableelectrically-conductive material faces the first side of the firstsub-cell and the first side of the second sub-cell. Alternatively or inaddition, the step of depositing may include depositing at least one ofconductive epoxy, wire mesh, or charge collection tape. Furthermore, themethod optionally comprises disposing a holographic element in opticalcommunication with at least one of the first and second sub-cellsbetween the optically-transparent encapsulation layer and a surface ofthe at least one of the first and second sub-cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a holographic planar concentrator.

FIG. 2A shows an embodiment of a holographic spectrum-splitting device.

FIG. 2B shows an alternative embodiment of a holographicspectrum-splitting device.

FIG. 3 is a schematic cross sectional diagram showing the arrangement oftwo bifacial solar cells arranged according to an embodiment of theinvention

FIG. 4 is a top-down plan view of an arrangement of bifacial solar cellsmutually electrically coupled according to an embodiment of theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

References throughout this specification to “one embodiment,” “anembodiment,” “a related embodiment,” or similar language mean that aparticular feature, structure, or characteristic described in connectionwith the referred to “embodiment” is included in at least one embodimentof the present invention. Thus, appearances of the phrases“in oneembodiment,” “in an embodiment,” and similar language throughout thisspecification may, but do not necessarily, all refer to the sameembodiment. It is to be understood that no portion of disclosure, takenon its own and in possible connection with a figure, is intended toprovide a complete description of all features of the invention.

In addition, the following disclosure may describe features of theinvention with reference to corresponding drawings, in which likenumbers represent the same or similar elements wherever possible. In thedrawings, the depicted structural elements are generally not to scale,and certain components are enlarged relative to the other components forpurposes of emphasis and understanding. It is to be understood that nosingle drawing is intended to support a complete description of allfeatures of the invention. In other words, a given drawing is generallydescriptive of only some, and generally not all, features of theinvention. A given drawing and an associated portion of the disclosurecontaining a description referencing such drawing do not, generally,contain all elements of a particular view or all features that can bepresented is this view, for purposes of simplifying the given drawingand discussion, and to direct the discussion to particular elements thatare featured in this drawing. A skilled artisan will recognize that theinvention may possibly be practiced without one or more of the specificfeatures, elements, components, structures, details, or characteristics,or with the use of other methods, components, materials, and so forth.Therefore, although a particular detail of an embodiment of theinvention may not be necessarily shown in each and every drawingdescribing such embodiment, the presence of this detail in the drawingmay be implied unless the context of the description requires otherwise.In other instances, well known structures, details, materials, oroperations may be not shown in a given drawing or described in detail toavoid obscuring aspects of an embodiment of the invention that are beingdiscussed. Furthermore, the described single features, structures, orcharacteristics of the invention may be combined in any suitable mannerin one or more further embodiments.

For example, to simplify a particular drawing of an electro-opticaldevice of the invention not all coatings or layers (whether electricallyconductive, reflective, or absorptive or other functional coatings suchas alignment coatings or passivation coatings), electricalinterconnections between or among various elements or coating layers,elements of structural support (such as holders, clips, supportingplates, or elements of housing, for example), or auxiliary devices (suchas sensors, for example) may be depicted in a single drawing. It isunderstood, however, that practical implementations of discussedembodiments may contain some or all of these features and, therefore,such coatings, interconnections, structural support elements, orauxiliary devices are implied in a particular drawing, unless statedotherwise, as they may be required for proper operation of theparticular embodiment.

Moreover, if the schematic flow chart diagram is included, it isgenerally set forth as a logical flow-chart diagram. As such, thedepicted order and labeled steps of the logical flow are indicative ofone embodiment of the presented method. Other steps and methods may beconceived that are equivalent in function, logic, or effect to one ormore steps, or portions thereof, of the illustrated method.Additionally, the format and symbols employed are provided to explainthe logical steps of the method and are understood not to limit thescope of the method. Although various arrow types and line types may beemployed in the flow-chart diagrams, they are understood not to limitthe scope of the corresponding method. Indeed, some arrows or otherconnectors may be used to indicate only the logical flow of the method.For instance, an arrow may indicate a waiting or monitoring period ofunspecified duration between enumerated steps of the depicted method.Without loss of generality, the order in which processing steps orparticular methods occur may or may not strictly adhere to the order ofthe corresponding steps shown.

The invention as recited in claims appended to this disclosure isintended to be assessed in light of the disclosure as a whole, includingfeatures disclosed in prior art to which reference is made.

A “laminate” refers generally to a compound material fabricated throughthe union of two or more components, while a term “lamination” refers toa process of fabricating such a material. Within the meaning of the term“laminate,” the individual components may share a material composition,or not, and may undergo distinct forms of processing such as directionalstretching, embossing, or coating. Examples of laminates using differentmaterials include the application of a plastic film to a supportingmaterial such as glass, or sealing a plastic layer between twosupporting layers, where the supporting layers may include glass,plastic, or any other suitable material.

As broadly used and described herein, the reference to an electrode orlayer as being “carried” on a surface of an element refers to bothelectrodes or layers that are disposed directly on the surface of anelement or disposed on another coating, layer or layers that aredisposed directly on the surface of the element.

An HPC 100, shown schematically in a cross-sectional view in FIG. 1,typically includes a highly-transparent planar substrate 104 ofthickness d (such as, for example, substrate made of glass orappropriate polymeric material having the refractive index n₁) at leastone diffractive structure 108, having width t, at a surface of thesubstrate 104. Such diffractive structure may include, for example, aholographic optical film (such as gelatin-on-PET film stack) in which aplurality of multiplexed diffraction gratings have been recorded withthe use of laser light. The diffractive structure 108 can be optionallycapped with a protective cover layer (not shown). The substrate 104 istypically cooperated with a solar-energy-collecting device 112 such as aPV cell. The diffractive structures 108 diffract wavelengths usable bythe PV cell 112, while allowing the light at unusable wavelength to passthrough, substantially unabsorbed. The usable energy is guided via thetotal internal reflection at the glass/air or glass/cover interface tostrings of solar cells, resulting in up to a 3× concentration of solarenergy per unit area of PV material.

As shown in FIG. 1, the PV cell 112 of width T is juxtaposed with thesecond surface of the substrate 104 in opposition to the diffractivestructures 108 and in such orientation that ambient (sun-) light I,incident onto the structure 108 at an angle θ_(I), is diffracted at anangle θ_(D) onto the cell 112 either directly or upon multiplereflections within the substrate 104. To estimate the range of incidentangles that would produce the diffracted light intersecting the surfaceof the PV cell 112 for different parameters of the HPC 100 such assubstrate thickness, the displacement of the PV cell 112 with respect tothe edge of the grating 108, other geometrical parameters one can usethe grating equation. For example, for a glass substrate 104 and whent=T=d, the range of incident angles (the collection angle) at which thecell 112 is illuminated is about 45 degrees. When t=2T−2d, thecollection angle is reduced to about 38 degrees. The angular rangewithin which the corresponding diffracted light is produced is about 10°to 15° for most of the wavelengths. However, the angle-wavelengthmatching can be used to extend this range for different portions of theavailable spectral bandwidth of the HPC 100.

The increase in PV-conversion efficiency, in comparison with a use of aconventional PV-cell, is also achieved by using multiple junction cellsthat create electron-hole pairs at the expense of energy of incidentlight over a wider spectral range than a single junction cell. The useof holographic grating with such spectrum-splitting devices (SSD) alsooffers some advantages. The hologram can be designed to diffract lightwithin a specific spectral band in a desired direction (for example,towards one PV-cell) and be multiplexed with another hologram thatdiffracts light of different wavelength in another direction (forexample, towards another PV-cell). One example of such holographic SSD200, shown in FIG. 2A, includes two holographically-recorded diffractivestructures 204 and 208 that are cascaded at a surface 212 of thesubstrate 216 (i.e., at the input of the SSD 200) and that diffractlight of different wavelengths. For example, the upper hologram 204diffracts light at wavelength λ₂ longer than wavelength λ₂ diffracted bythe hologram 208. Two PV-cells, respectively-corresponding to theholograms 204 and 208—a long-wavelength PV cell 214 and ashort-wavelength PV-cell 218—are positioned transversely with respect tothe holograms 204, 208 (as shown, at side facets of the substrate 216).Directionally-diffracted towards target PV-cells light 224, 228 reachesthe PV-cells via reflections off the surfaces of the substrate 216. Asimple light-concentrating reflector can additionally be used. A similarSSD 230, upgraded with cylindrical parabolic reflectors 234, 238 thatguide the diffracted light towards target PV-cells, is depicted in FIG.2B. In both cases, the collection angle is determined by geometry of thesystem and the diffraction characteristics of the holograms.

Bifacial solar cells are known to have unequal efficiencies of solarenergy conversion for the front and back sides of an individual PVcells. It is unexpectedly discovered that, when such PV cells areassembled into conventional panels or series such that the “front” sides(high efficiency sides) of all the cells are oriented to interceptdirect sunlight, while the lower efficiency or “back” sides are orientedto receive sunlight delivered indirectly (from scatter, reflection offof the ground, or mounting surface, for example; see examples of FIGS.2A, 2B above), the electrical energy output from the resulting panels isnot optimized.

Unfortunately, it is precisely such non-optimized orientation that isused in fabrication of commercially-available PV modules or panels:Because of the stack-orientation for bifacial cells, the polarity ofsome bifacial cells is such that the positive pole occurs at the highefficiency side. Accordingly, even though the cells are bifacial innature they are still strung in a conventional manner in the module,with the highest efficiency side facing the sun, resulting in all thecell polarities being located on the same side.

The idea of the present invention stems from the realization thatelectrical connection among the individual PV cells in a stack arrangedsuch that the back side of a first cell electrically connected to thefront side of the adjacent cell, or vice versa, increases the efficiencyof harvesting the solar energy.

An arrangement of mutually electrically coupled bifacial solar cells isshown in cross section in FIG. 3. Each bifacial cell has a positive andnegative poll (electrically-positive and electrically-negative sides).In the example of FIG. 3, the positive poll of cell 300 is situated atthe “top” active surface, and the negative poll of cell 300 is situatedat the “bottom” active surface. The polls of cell 316 are oriented inthe opposite manner.

As shown in FIG. 3, the bifacial solar cells 300 and 316 are arrangedsuch that the electrically-positive face of solar cell 300 issubstantially coplanar with the electrically-negative face of adjacentsolar cell 316. Bifacial solar cells 300 and 316 may be equal efficiencybifacial cells. Where bifacial cells 300, 316 are equal efficiencycells, each of the cells 300, 316 has a front and back active surfaces,where each active surface demonstrates substantially the same level ofsolar conversion efficiency. In such special case, and from anefficiency standpoint, it does not matter which surface—the one with anegative electrical polarity or the one with a positive electricalpolarity—is oriented toward a source of direct illumination.

Each bifacial solar cell 300, 316 is constructed of multiple layers ofdoped semiconductor material. For example, the cell 300 has a substratelayer 302 sandwiched between differently doped semiconductor layers 304,306. In one example, the layer 302 is a p-type semiconductor substratelayer, the layer 304 is an n+-type semiconductor material and the layer306 is a p+-type semiconductor material. The layers 302, 304, 306 areoptionally formed of single-crystalline, poly-crystalline or amorphoussilicon. Other semiconductor layer stacks can be used to create abifacial photovoltaic cell, as long as the requirement that thesemiconductor stack be capable of producing electron-hole pairs at eachjunction between its layers (the junction between the layers 304 and302, and the junction between the layers 302 and 306, in case of thecell 300) when light is incident on a front side and a back side of thecell is satisfied. The bifacial cell 316 is similar to cell 300, butoriented such that its negative pole is facing the same direction and,optionally, is also co-planar with the side of the cell 300 associatedwith the positive poll. Like cell 300, cell 316 has a substrate layer318 sandwiched between two differently doped semiconductor layers 320,322. In an exemplary embodiment, layer 318 is a p-type substrate layer,layer 322 is an n+-type semiconductor material and layer 320 is ap+-type semiconductor material.

On the outside of the active semiconductor layers of the cells 300 and316 additional layers may be present (shown, in FIG. 3, as layers 308,310, 326, and 324). Each of these additional layers may carry or besupplemented by at least one auxiliary layer (not shown) such as, forexample, a transparent electrode layer (made of a transparent conductiveoxide, TCO, such as indium tin oxide ITO or aluminum zinc oxide AZO, forexample), a passivation coating, an anti-reflection coating, and a hardprotective coatings, to name just a few

The cell 300 is illustrated to be equipped with electrical contacts 312and 314 that are electrically coupled to semiconductor layers 308 and310, respectively. Such electrical coupling may be effectuated bysoldering, for example, or printing the contacts 312, 314 at the TCOlayer (not shown) juxtaposed with the layers 308 and 310 respectively.Electrical contacts 330, 328 are provided in a similar fashion for theadjacent cell 316 in electrical cooperation with the layers 324, 326.

The individual PV cell such as the cell 300, when illuminated withsunlight, generally generates a relatively low level of electricpotential, for example about 0.5 V. Accordingly, to optimize the solarenergy harvesting, the cell 300 is preferably connected with additionalPV cells to form modules generating higher and more practically-usefulvoltage levels. The arrangement of FIG. 3, showing two electricallyconnected individual PV cells 300, 316, may be only a small segment of atypical solar module. As shown an electrically-positive side of cell 300is connected to an electrically-negative side of the adjacent cell 316via an electrical buss connection 332. The negative side of cell 300 isconnected, as shown with an arrow 333, to an electrically-positive sideof an adjacent non-illustrated cell by bus connection 334. Unlikeconventional bus connections, which connect the back side of a firstcell to the front side of an adjacent cell, the connections of theembodiment of FIG. 3 332, 334 are co-planar with their respectiveconnected surfaces.

The arrangement of FIG. 3 additionally shows an optional encapsulantlayer 336, which in some embodiments is bonded to or deposited on theoutside of the outermost layers 308, 310, 324, 326 of cells 300, 316.While in the arrangement of FIG. 3, only a single encapsulant layer 336is shown, in a related embodiment the encapsulant may be disposed overseveral sides of the PV cell or the PV module containing several PVcells that are connected according to the idea of the invention such asto embed the PV module within the encapsulant. In certain embodiments,the encapsulant layer 336 is formed of a sheet of EVA that is laminatedto solar cells 300, 316 in order to provide enhanced mechanicaldurability and scratch resistance, for example. In so affixing theencapsulate sheet to the solar cells, the electrical connections 332,334 (i.e., bus bars) may be pre-deposited onto the surface of theencapsulant layer 336 that would be facing the PV cells prior toassembly or lamination of the encapsulant layer 336 onto the cells 300,316. This pre-deposition of electrical connectors 332, 334 is enabledand made practical precisely because of the mutual orientation of theindividual solar cells 300, 316 in accord with an embodiment of theinvention, as such orientation (shown in FIG. 3) obviates the need forfront-to-back electrical coupling between adjacent cells that would berequired in the case of conventional orientation of the cells. As theelectrical connectors 332, 334 are substantially parallel with the frontand back surfaces of the cells 300, 316, these connectors can bepre-disposed onto the encapsulant layer 336, which is also substantiallyco-planar with the surfaces of cells 300, 316 and, therefore, attachedto the PV cells simultaneously and in the same processing step withattachment of the encapsulant layer 336 to the cells.

In a related embodiment, additional cells are connected in series to thecells 330, 316 in a similar manner, by orienting and arranging theseadditional cells such that both the front side and the back side of theresulting module is built of cells having alternating positive andnegative polls. An example of the resulting PV module 400 is shown inFIG. 4, which shows in a top plan view a two-dimensional array ofbifacial solar cells mutually electrically coupled according to anembodiment of the invention. In the embodiment 400, thirty six (36)equal efficiency bifacial cells (e.g., 401 through 436) are provided.Each cell has positive and a negative face. For example, cell 401 isoriented such that its positive face points “up” out of the xy-plane ofFIG. 4. Similarly, cell 402 is oriented such that its negative facepoints “up” out of the plane of FIG. 4. Individual cells of theembodiment 400 are configured in a string with cells having alternateelectrical polarity facing the same direction. Adjacent cells areelectrically serially connected with bus bars. Because of the alternatepolarity arrangement, bus bars are adapted to electrically connectadjacent cells in a front-to-front, and back-to-back manner, rather thanin a conventional back-to-front manner of the related art. In theembodiment 400, a bus bar 444 connects the positive face of cell 401with the substantially co-planar negative face of adjacent cell 402. Thepositive face cell 402 is connected to the negative face of the adjacentcell 403 in the string by a bus bar 446, and so on, to result in amultiple serial connection of individual solar cells. The entire stringof cells supplies its combined output current through bus bar 448.

According to an embodiment of the invention, high efficiency individualPV cells, whether or not bifacial, are reduced in area as compared tothe size of the conventional PV cells. This may be achieved by dicing orscribing-and-breaking off-the-shelf available PV cells. An individualreduced-area cell generates proportionately reduced amount of current.For example, a module constructed of 125×125 mm² uncut PV cells with asingle PV cell solar conversion efficiency of about 22% will have anormal operating current of about 3.5 A to about 4 A. As the size of anindividual cell is increased to, for example, 150×150 mm², the currentprovided by such individual PV cell under otherwise identical conditionswill increase to about 5-5.8 A while the voltage remains constant.Resistive power losses associated with a solar cell are proportional tothe product of the series resistance with the square of the modulecurrent and can be assessed as P_(LOSS)=I²R_(SERIES).

According to an embodiment of the invention, large cells are cut intosmaller elements, thereby effectively reducing the current whilemaintaining the same voltage. In one embodiment, a cell is cut intothree equal sized sections, reducing the current to a ⅓ of the currentprovided by the originally-size cell. If the electrical resistances in amodule are maintained, the reduction of a per-cell output current willeffectuate an approximate 9 time reduction in electrical power lost dueto dissipation on resistance.

In one example, the individual-cell area-reduction approach wasimplemented with a Sanyo 180 W PV module. Conventionally, such PV modulegenerates about 3.3 A of current at 1.19 Ohm resistance, therebyproducing about 13 W in resistive losses. A Sanyo 180 W module was dicedinto three parts each having ⅓rd the area of the original module. Theresult was three smaller modules each generating ⅓rd the current of theoriginal module. The electrical assembly of individual smaller modulesaccording to an embodiment of the invention exhibited only about 1/9thof the resistive losses of the original module.

Methods according to embodiments of the invention include reducing thearea of solar cells or modules to form sub-cells, followed by applyingbus bar or similar electrical connectors made of higher resistivity, butotherwise similar, materials to connect the reduced-area sub-cells. Forexample, according to embodiments of the invention, conductive epoxy,wire mesh, charge collection tape “charge tape”, flex circuits/flexiblePCBs, or other flexible, or more easily assembled bus structures can beused without incurring high resistive losses. In one embodiment, busstructures such as beads of electrically conductive epoxy are applied toan encapsulation film (such as the layer 336 of FIG. 3) prior to theassembly or lamination of the encapsulation film to the diced, reducedarea solar sub-cells. In a related embodiment, a charge tape and/ormetallic wire mesh was applied to the partially melted or softenedencapsulation film prior to the assembly or lamination of theencapsulation film to the diced, reduced area solar sub-cells.

Embodiments according to the invention provide unexpected results thatare advantageous over the related art. Bussing together bifacial solarcells in the same plane, without back-to-front or front-to-backconnections as done in the related art, simplifies assembly, reduces theamount of handling to which each cell is subjected, and accordingly,reduces the chance of breakage and associated cost. Specifically,arrangements of equal efficient bifacial cells eliminate theconventional need that extra care must be taken not to confuse the frontand back sides of cells when constructing a module, to en sure that thehigh efficiency side of the cells faces the sun. Additionally, the useof in-plane, front-to-front and back-to-back bussing, as opposed toconventional back-to-front/front-to-back bussing, simplifies assembly,makes open circuits easier to detect and repair, and results in a morerobust package.

Additionally, by cutting/dicing bifacial cells into smaller pieces, thecurrent associated with individual cell is reduced in proportion to thedecrease in area of the cell. Reduced current allows for the use of busconductors having higher resistance. For example, with low current, theuse of charge tape, conductive epoxy, wire meshes, and other higherresistance bussing materials becomes possible. These bussing materialshave certain advantages such as increased flexibility, ease ofinstallation, lower cost, and increased durability. In particular,bussing materials can be added to encap sulant layers (EVA, Surlyn,etc.) in predetermined patterns by direct deposition in the case of theepoxy or a partial melt in the case of charge tape and wire meshes. Thebussing material pattern can be pre-deposited to match the layout of thecells so as to fully contact the cell bus bars and fingers. The patternmay be deposited on the encapsulant layers on both sides of the cellplane with their corresponding pattern. In alternative embodiments,where the active semiconductor material of the solar cell is embedded ina waveguide, bussing material may be pre-deposited on one surface of thewaveguide, which is later adhered to or index matched with the cell.

The invention has been described with reference to certain specificembodiments. Those skilled in the art of mine management and distributedcomputing systems generally may develop other embodiments of the presentinvention. The terms and expressions that have been used to describecertain embodiments in the foregoing specification are terms ofdescription, rather than limitation, and, in using such terms, there isno intention to exclude equivalents of the features shown and described.Various configurations of individual PV cells (for example, cellsincluding holograms), cooperation of PV modules in series of PV modulesvia flexible joints, and additional features of electrical connectorsproviding electrical communications between individual PV cells and/orindividual PV modules of a series of the PV modules are discussed inabove-mentioned patent applications incorporated herein by reference intheir entirety. It should be apparent that modifications and adaptationsto those embodiments may occur to one skilled in the art withoutdeparting from the scope of the present invention.

What is claimed is:
 1. A solar module comprising: a first bifacial solarcell having a first front active side and a first back active side,wherein the first front and back active sides have substantially equalsolar conversion efficiencies, the first front active side having afirst electrical polarity, the first back active side having a secondelectrical polarity that is opposite to the first electrical polarity; asecond bifacial solar cell having a second front active side and asecond back active side, wherein the second front and back active sideshave substantially equal solar conversion efficiencies, the second frontactive side having the second electrical polarity and the second backactive side having the first electrical polarity, the first and secondbifacial solar cells oriented in series such that the first front activeside is facing in substantially the same direction as the second frontactive side, and a bus bar electrically coupling the first front activeside to second front active side.
 2. A solar module according to claim1, wherein the first and second bifacial solar cells are arrangedadjacent to one other, and the first front active side is substantiallycoplanar with the second front active side.
 3. A solar module accordingto claim 1, further comprising a third bifacial solar cell having athird front active side and a third back active side, wherein the secondfront and back active sides have substantially equal solar conversionefficiencies, the third front active side having the first electricalpolarity, the third back active side having the second electricalpolarity, the third bifacial solar cell oriented such that the thirdfront active side faces substantially the same direction as the firstfront active side; and a bus bar electrically coupling the second backactive side to the third back active side.
 4. A solar module accordingto claim 1, further comprising an encapsulant layer disposed on thefirst front active side and the second front active side such as tocover the bus bar.
 5. A method for fabrication of a photovoltaic (PV)module, comprising: separating a PV cell having an original size into aplurality of PV sub-cells, each sub-cell having a size smaller than theoriginal size; electrically coupling the sub-cells in series such that afirst side of the first sub-cell having a first electrical polarity iselectrically connected to a first side of the second sub-cell having asecond electrical polarity, the first side of the first sub-cell and thefirst side of the second sub-cell oriented to face substantially thesame direction.
 6. A method according to claim 5, wherein the first sideof the firs sub-cell is positioned to be substantially co-planar withthe first side of the second sub-cell.
 7. A method according to claim 5,wherein said electrically coupling includes depositing a conformableelectrically conductive material on a first surface of anoptically-transparent encapsulation layer; and covering the first andsecond sub-cells with said optically-transparent encapsulation layersuch that the first surface of the optically transparent encapsulationlayer carrying the conformable electrically-conductive material facesthe first side of the first sub-cell and the first side of the secondsub-cell.
 8. A method according to claim 7, wherein said depositingincludes depositing at least one of conductive epoxy, wire mesh, orcharge collection tape.
 9. A method according to claim 5, furthercomprising disposing a holographic element in optical communication withat least one of the first and second sub-cells between theoptically-transparent encapsulation layer and a surface of the at leastone of the first and second sub-cells.